Long story short in my case:
Logged low fuel rail pressure (Thanks MacMan), so replaced:
LPFP - in tank, side pressure sensor on HPFP and new fuel filter (the **** that comes out there
), cam follower bucket.. Cleared logs after that, car was a bit more responsive but still bogged on WOT.
Upon closer inspection I saw that the plunger shaft was BAAAADLY worn ( about 1 mm
- will see if I can get pics upload of this. (my guess is this) Take off your HPFP again and inspect if its worn or not? I got a secondhand replacement one from Ruan (Thanks!)
Cleared logs again and no more issues. Unfortuneately this is a design flaw and a serviceable part from VW imho. Best to check this every 15000KM. Any wear on plunger, cambucket or follower will result in loss of performance guaranteed
Post some pics of what it looks like.